Method and apparatus for multi-channel downhole electromagnetic telemetry

ABSTRACT

An electromagnetic (EM) telemetry method comprises encoding downhole data into a single data stream; separating the single data stream into a plurality of separate data streams; converting each separate data stream into a corresponding separate waveform using a selected digital modulation technique wherein at least one of the frequency and phase of each waveform is assigned a unique value or unique non-overlapping range of values; combining each separate waveform into a combined waveform; and transmitting from a downhole location, an electromagnetic (EM) telemetry carrier wave comprising the combined waveform.

FIELD

This invention relates generally to downhole measurement-while-drilling(MWD) using electromagnetic (EM) telemetry, and in particular to amethod and apparatus for transmitting and receiving multi-channeldownhole EM telemetry.

BACKGROUND

The recovery of hydrocarbons from subterranean zones relies on theprocess of drilling wellbores. The process includes drilling equipmentsituated at surface and a drill string extending from the surfaceequipment to the formation or subterranean zone of interest. The drillstring can extend thousands of feet or meters below the surface. Theterminal end of the drill string includes a drill bit for drilling (orextending) the wellbore. In addition to the conventional drillingequipment mentioned, the system also relies on some sort of drillingfluid system, in most cases a drilling “mud” which is pumped through theinside of the pipe, which cools and lubricates the drill bit and thenexits out of the drill bit and carries the rock cuttings back tosurface. The mud also helps control bottom hole pressure and preventhydrocarbon influx from the formation into the wellbore which canpotentially cause a blow out at surface.

Directional drilling is the process of steering a well away fromvertical to intersect a target endpoint or follow a prescribed path. Atthe terminal end of the drill string is the bottom-hole-assembly (orBHA) which comprises of 1) drill bit; 2) steerable downhole mud motor ofrotary steerable system; 3) sensors of survey equipment (Logging WhileDrilling (LWD) and/or Measurement-while-drilling (MWD)) to evaluatedownhole conditions as drilling progresses; 4) equipment for telemetryof data to surface; and 5) other control process equipment such asstabilizers or heavy weight drill collars. The BHA is conveyed into thewellbore by a string of metallic tubulars (drill pipe). MWD equipment isused to provide downhole sensor and status information to surface in anear real-time mode while drilling. This information is used by the rigcrew to make decisions about controlling and steering the well tooptimize the drilling speed and trajectory based on numerous factors,including lease boundaries, existing wells, formation properties,hydrocarbon size and location, etc. This can include making intentionaldeviations from the planned wellbore path as necessary based on theinformation gathered from the downhole sensors during the drillingprocess. The ability to obtain real time data MWD allows for arelatively more economical and more efficient drilling operation.

In MWD, the currently used MWD tools contain essentially the same sensorpackage to survey the well bore but send the data back to surface byvarious telemetry methods. Such telemetry methods include but are notlimited to the use of hardwired drill pipe, acoustic telemetry, fibreoptic cable, Mud Pulse (MP) Telemetry and Electromagnetic (EM)Telemetry.

EM Telemetry involves the generation of electromagnetic waves whichtravel through the wellbore's surrounding formations, with detection ofthe waves at surface. The BHA metallic tubular is typically used as thedipole antenna for the EM telemetry tool by dividing the drill stringinto two conductive sections by an insulating joint or connector (“gapsub”) typically placed within the BHA, with the bottom portion of theBHA and the drill pipe above each forming a conductor for the dipoleantenna. In EM telemetry systems, a very low frequency alternatingcurrent is driven across the gap sub. The sub is electrically isolated(‘nonconductive”) at its center joint, effectively creating aninsulating break (“gap”) between the very bottom of the drill string andthe larger top portion that includes all the drill pipe up to thesurface. The low frequency AC voltage and magnetic reception iscontrolled in a timed/coded sequence to energize the earth and create ameasureable voltage differential between the surface ground and the topof the drill string. The EM signal which originated across the gap isdetected at surface and measured as a difference in the electricpotential from the drill rig to various surface grounding rods locatedabout the lease site.

Advantageously, an EM system can transmit data without a continuousfluid column; hence it is useful when there is no mud flowing. This isadvantageous because the EM signal can transmit the directional surveydata while the drill crew is adding new pipe.

However, EM transmissions can be strongly attenuated over long distancesthrough the earth formations, with higher frequency signals attenuatingfaster than low frequency signals, and thus EM telemetry tends torequire a relatively large amount of power so that the signals can bedetected at surface.

MWD telemetry methods rely on modulation of digital signals similar tothat developed in the telecommunications industry. Typically, the signalis modulated by a variety of standard modulation techniques. The threekey parameters of a periodic waveform are its amplitude (“volume”), itsphase (“timing”) and its frequency (“pitch”). Any of these propertiescan be modified in accordance with a low frequency signal to obtain themodulated signal. Frequency-shift keying (FSK) is a frequency modulationscheme in which digital information is transmitted through discretefrequency changes of a carrier wave. The simplest FSK is binary FSK(BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0sand 1s) information. Amplitude shift keying (ASK) conveys data bychanging the amplitude of the carrier wave; Phase-shift keying (PSK)conveys data by changing, or modulating, the phase of a reference signal(the carrier wave). It is known to combine different modulationtechniques. For example, combining Amplitude and Phase-shift keying is adigital modulation scheme that conveys data by changing, or modulating,both the amplitude and the phase of a reference signal (or the carrierwave). Asymmetric Phase-shift keying, (APSK), combines bothAmplitude-shift keying (ASK) and Phase-shift keying (PSK) to increasethe symbol-set.

The choice of modulation scheme uses a finite number of distinct signalsto represent digital data. PSK uses a finite number of phases, eachassigned a unique pattern of binary digits. Usually, each phase encodesan equal number of bits. Each pattern of bits forms the symbol that isrepresented by the particular phase. The demodulator, which is designedspecifically for the symbol-set used by the modulator, determines thephase of the received signal and maps it back to the symbol itrepresents, thus recovering the original data. This requires thereceiver to be able to compare the phase of the received signal to areference signal.

SUMMARY

According to one aspect of the invention, there is provided anelectromagnetic (EM) telemetry method comprising: encoding downhole datainto a single data stream; separating the single data stream into aplurality of separate data streams; converting each separate data streaminto a corresponding separate waveform using a selected digitalmodulation technique wherein at least one of the frequency and phase ofeach waveform is assigned a unique value or unique non-overlapping rangeof values; combining each separate waveform into a combined waveform;and transmitting from a downhole location, an electromagnetic (EM)telemetry carrier wave comprising the combined waveform. The method canfurther comprise: receiving the carrier wave at a surface location,amplifying the carrier wave, applying a band-pass filter to the carrierwave, and filtering each separate waveform from the carrier wave usingthe assigned unique value of that separate waveform; demodulating eachseparate waveform into the corresponding separate data stream; andcombining the separate data streams into the single data stream. Thus,the single data stream can be decoded back into the downhole data anddisplayed.

The selected digital modulation technique can be selected from the groupconsisting of: amplitude shift keying (ASK), phase shift keying (PSK),and frequency shift keying (FSK). In particular, the digital modulationtechnique can be PSK (either BFSK or QFSK) and only the frequency ofeach waveform can be assigned a unique value.

The carrier wave can be an analog signal and the step of demodulatingcan be applied to the analog carrier wave in which case the separatedata streams are analog and are converted into digital data streams.Alternatively, the carrier wave can be analog and the method can furthercomprise converting the analog carrier wave into a digital signal beforethe separate data streams are separated from the carrier wave.

According to another aspect of the invention, there is provided anelectromagnetic (EM) telemetry system comprising a downhole telemetrytool that includes: a gap sub assembly; an EM carrier frequency signalgenerator for generating an EM carrier wave across an electricallyisolated gap of the gap sub assembly; and an electronics subassemblycommunicative with the signal generator. The electronics subassemblycomprises a downhole processor and a memory containing an encoderprogram code. This encoded program code is executable by the downholeprocessor to perform a method comprising: encoding downhole data into asingle data stream; separating the single data stream into a pluralityof separate data streams; converting each separate data stream into acorresponding separate waveform using a selected digital modulationtechnique wherein at least one of the frequency and phase of eachwaveform is assigned a unique value or unique non-overlapping range ofvalues; combining each separate waveform into a combined waveform; andsending control signal to the signal generator to transmit an EMtelemetry carrier wave comprising the combined waveform.

The downhole telemetry tool can further comprise a directional andinclination sensor module and a drilling conditions sensor module thatare both communicative with the processor.

The system can also comprise a surface receiver configured to receivethe carrier wave; and a decoder communicative with the surface receiver.The decoder comprises a surface processor and a memory containing adecoder program code executable by the surface processor to perform amethod comprising: filtering each separate waveform from the carrierwave using the assigned unique value of that separate waveform;demodulating each separate waveform into the corresponding separate datastream; and combining the separate data streams into the single datastream. The decoder program can be further executable by the surfaceprocessor to decode the single data stream back into the downhole dataand to transmit the downhole data to a display.

The surface receiver can further comprise an amplifier configured toamplify the received carrier wave and a band pass filter configured tofilter out unwanted noise in the received carrier wave.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is schematic side view of a multi-channel EM telemetry system inoperation, according to embodiments of the invention.

FIG. 2 is a schematic block diagram of components of a downhole EMtelemetry tool of the EM telemetry system according to one embodiment.

FIG. 3 is a schematic block diagram of components an electronicssubassembly of the downhole EM telemetry tool.

FIG. 4 is a flow chart of steps performed by the downhole EM telemetrytool in a method for transmitting a multi-channel EM telemetry signalwith a combined waveform that is a combination of two or more separatewaveforms each representing a separate telemetry channel.

FIG. 5 is a schematic block diagram of surface components of themulti-channel EM telemetry system according to one embodiment.

FIG. 6 is a schematic block diagram of surface components of themulti-channel EM telemetry system according to another embodiment.

FIG. 7 is a flow chart of steps performed by the surface components ofthe multi-channel EM telemetry system to receive and decode themulti-channel EM telemetry signal transmitted by the downhole EMtelemetry tool.

FIG. 8 is a graph of a first downhole waveform of a first telemetrysignal.

FIG. 9 is a graph of a second downhole waveform of a second telemetrysignal.

FIG. 10 is a graph of a multi-channel EM telemetry signal that is thecombination of the first and second downhole waveforms, as transmittedby the downhole EM telemetry tool.

FIG. 11 is a graph of the multi-channel EM telemetry signal as receivedby the surface components.

FIG. 12 is a graph of first telemetry signal separated from the EMtelemetry signal by the surface components.

FIG. 13 is a graph of second telemetry signal separated from the EMtelemetry signal by the surface components.

FIG. 14 is a graph of a downhole reference frequency sweep waveformhaving a continuously increasing frequency according to one embodiment.

FIG. 15 is a graph of the downhole reference frequency sweep waveform ofFIG. 14 as received by a surface receiver.

FIG. 16 is a graph plotting the amplitude against frequency range of thereceived downhole frequency sweep waveform of FIG. 15.

FIG. 17 is a graph of a downhole reference frequency sweep waveformhaving discreet frequency steps according to another embodiment.

FIG. 18 is a graph of the downhole reference frequency sweep waveform ofFIG. 17 as received by a surface receiver.

FIG. 19 is a graph plotting the amplitude against frequency range of thereceived downhole frequency sweep waveform of FIG. 18.

DETAILED DESCRIPTION

Directional terms such as “top,” “bottom,” “upwards,” “downwards,”“vertically,” and “laterally” are used in the following description forthe purpose of providing relative reference only, and are not intendedto suggest any limitations on how any article is to be positioned duringuse, or to be mounted in an assembly or relative to an environment.

An EM signal generated by a downhole EM signal transmitter needs to havea sufficient strength that the signal is still detectable at surface bya surface EM signal receiver despite considerable attenuation of thetransmitted signal as the signal travels long distances through theearth's formations.

The embodiments described herein generally relate to a multi-channel EMtelemetry system which comprise a downhole EM telemetry tool thatcombines multiple EM waveforms each representing a separate channel oftelemetry data into a combined waveform and transmits this combinedwaveform to surface in a single EM transmission, and which alsocomprises a surface receiver that receives the combined waveform anddecodes the waveform into the separate channels of telemetry data. It isexpected that a high overall efficiency of data transmission can beachieved by sending multiple channels of telemetry data in a single EMtransmission comprising the combined waveform.

Referring to FIG. 1, there is shown a schematic representation of an EMtelemetry system in which various embodiments can be employed. Downholedrilling equipment including a derrick 1 with a rig floor 2 and drawworks 3 facilitate rotation of drill pipe 6 into the ground 5. The drillpipe 6 is enclosed in casing 8 which is fixed in position by casingcement 9. Bore drilling fluid 10 is pumped down drill pipe 6 and throughan electrically isolating gap sub assembly 12 to drill bit 7. Annulardrilling fluid 11 is then pumped back to the surface and passes througha blow out preventer (BOP) 4 positioned above the ground surface. Thegap sub assembly 12 may be positioned at the top of the BHA, with theBHA and the drill pipe 6 each forming a conductor of the dipole antenna.The gap sub assembly 12 is electrically isolated (nonconductive) at itscenter joint effectively creating an insulating break, known as a gap,between the bottom of the drill string with the BHA and the larger topportion of the drill string that includes the rest of the drill pipe 6up to the surface. A very low frequency alternating electrical current14 is generated by an EM carrier frequency signal transmitter 13 anddriven across the gap sub assembly 12. The low frequency AC voltage andmagnetic reception is controlled in a timed/coded sequence to energizethe earth and create an electrical field 15. Communication cables 17transmit the measurable voltage differential from the top of the drillstring and various surface grounding rods 16 located about the drillsite to a signal receiver box 18 which receives and processes the EMtelemetry transmission. The grounding rods 16 are generally randomlylocated on site with some attention to site operations and safety. Areceiver box communication cable 19 transmits the data received to acomputer display 20 after decoding, thereby providing measurement whiledrilling information to the rig operator.

Referring now to FIG. 2, an EM telemetry system 30 according to oneembodiment comprises a multi-channel downhole EM telemetry tool 32 andsurface receiving and processing equipment 34. The EM telemetry tool 32generally comprises the gap sub assembly 12, the EM carrier frequencysignal transmitter 13, and an electronics subassembly 36. Theelectronics subassembly 36 houses sensors for taking downholemeasurements as well as a processor and memory which contains programcode executable by the processor to encode the sensor measurements intomultiple channels of telemetry data each with a separate EM waveform,combine the separate EM waveforms into a combined waveform, and sendcontrol signals to the EM carrier frequency transmitter 13 to transmitthe combined waveform to surface. The surface receiving and processingequipment 34 can be housed in the receiver box 18 and comprise equipmentto receive the combined waveform, filter and process the waveform, anddecode the waveform into the telemetry data.

The gap sub assembly 12 comprises an electrically conductive femalemember 37 comprising a female mating section and an electricallyconductive male member 40 comprising a male mating section. The malemating section 42 is matingly received within the female mating sectionand electrically isolated therefrom by an electrical isolator 46. Theelectrical isolator 46 comprises electrical insulating material that ispositioned in between the male and female mating sections. Theelectrical isolator 46 thereby electrically isolates the male member 40from the female member 37 and the male member 40, female member 37 andelectrical isolator 46 together function as the gap sub assembly 12 forEM telemetry.

Referring to FIG. 3, the electronics subassembly 36 comprises a tubularhousing (not shown) and the following components housed inside thehousing: a directional and inclination (D&I) sensor module 50; drillingconditions sensor module 52; a main circuit board 54 containing a masterprocessing unit (MPU or otherwise referred to as the “downholeprocessor”) 56, a memory 58 having stored thereon program codeexecutable by the controller 56, and one or more power amplifiers 59;and a battery stack 60. The downhole processor 56 can be any suitableprocessor known in the art for EM tools, and can be for example, adsPIC33 series MPU. The power amplifiers 59 can be a power MOSFETH-bridge design configured to transmit data.

The D&I sensor module 50 comprises three axis accelerometers, three axismagnetometers and associated data acquisition and processing circuitry.Such D&I sensor modules are well known in the art and thus are notdescribed in detail here.

The electronics subassembly 36 includes sensors mounted and circuitryfor taking various measurements of borehole parameters and conditionsincluding gamma, temperature, pressure, shock, vibration, RPM, anddirectional parameters. Such sensor circuitry are also well known in theart and thus are not described in detail here.

The main circuit board 54 can be a printed circuit board with electroniccomponents soldered on the surface of the board 54. The main circuitboard 54 and the sensor modules 50, 52 are secured on a carrier device(not shown) which is fixed inside the electronics subassembly housing byend cap structures (not shown). The sensor modules 50, 52 are eachelectrically communicative with the main circuit board 54 and sendmeasurement data to the downhole processor 56.

As will be described below, the memory 58 contains encoder program codethat can be executed by the downhole processor 56 to perform a method ofencoding and transmitting a multi-channel EM telemetry signal using acombined waveform that is a combination of two or more separatewaveforms each representing a separate telemetry channel. Referring toFIG. 4, the downhole processor 56 reads raw measurement data from thesensor modules 50, 52 and encodes this raw data into an encoded digitalbitstream (block 70). Then the downhole processor 56 separates theencoded digital bitstream into two or more separate digital bitstreams,which in the embodiment shown in FIG. 4 comprises a first bitstream anda second bitstream (block 72). Then, the downhole processor 56 convertseach digital bitstream into a separate waveform, namely a first waveformand a second waveform (otherwise referred to as “first telemetrychannel” and “second telemetry channel”); this conversion involves usinga selected digital modulation technique to modulate the waveforms,wherein the frequency of each waveform being modulated is assigned aunique value or a unique, non-overlapping range of values, i.e. a valueor range of values that is different than the value or range of valuesof the corresponding parameter(s) of the other waveforms (block 74).Then the downhole processor 56 combines each separate waveform into acombined waveform (block 76), and then sends a control signal to thesignal generator 13 to transmit an EM telemetry signal comprising thecombined waveform (otherwise referred to as “carrier wave”) across thegap sub 12 (block 78).

Alternatively, the conversion of each digital bitstream into a separatewaveform can use a selected digital modulation technique to modulate thewaveforms wherein the phase of each waveform being modulated is assigneda unique value or a unique non-overlapping range of values. In yetanother alternative, the conversion step can involve assigning a uniquefrequency and a unique phase (or unique non-overlapping ranges of suchfrequencies and phases) to each waveform.

Various digital modulation techniques known in the art can be used toencode each separate waveform, such as ASK, PSK, FSK, BPSK, QPSK or anycombination of these or other individual modulation techniques as isknown in the art. As one (or more) of the amplitude, frequency and phaseof the separate waveforms is set at a unique value, the plurality ofseparate waveforms encoded by one or more of these techniques can besuperimposed to form one combined waveform (carrier wave) fortransmission to surface. As a result, one EM signal comprising thecarrier wave composed of two or more telemetry channels can betransmitted to surface.

In one example and referring to FIGS. 8 to 10, each separate waveformcan be modulated using BPSK with different frequencies then combinedinto the combined waveform. FIG. 8 shows a first downhole waveform forthe first channel, having a maximum amplitude of 1.0, a frequency of 6Hz, and a phase shift of 180 degrees at time 0.5. FIG. 9 shows a seconddownhole waveform for the second channel, having a maximum amplitude of1.0, a frequency of 12 Hz, and a phase shift of 180 degrees at time 0.5.FIG. 10 shows a combined downhole waveform representing the combinationof the first and second downhole waveforms.

In another example (not shown), each separate waveform can be modulatedusing ASK with different frequencies. In this modulation technique, thepower amplifiers 59 can be used to modulate the amplitudes of eachwaveform instead of or in addition to the processor 56 performing adigital ASK modulation.

Referring now to FIG. 5, the surface receiving and processing equipment34 receives the carrier wave and decodes the combined waveform torecover each separate telemetry channel; the carrier wave sent by the EMtelemetry tool 32 shown in FIG. 10 will have attenuated as the carrierwave travels through the Earth and FIG. 11 shows the carrier wave asreceived by surface receiving and processing equipment 34. The telemetrychannels can then be converted back into the measurement data for use bythe operator. As will be described in detail below, the surfacereceiving and processing equipment 34 will have stored thereon ademodulation technique corresponding to the selected modulationtechnique used by the EM telemetry tool 32 and the unique phase orfrequency value of each separate waveform of the carrier wave used bythe downhole EM telemetry tool 32 to encode the separate waveforms, sothat the carrier wave can be decoded to obtain the telemetry data.

The surface receiving and processing equipment 34 comprises a surfacereceiver 80 and a decoder 82. The surface receiver 80 is located in thereceiver box 18 and comprises a preamplifier 84 electrically coupled tothe communication cables to receive and amplify the EM telemetrytransmission comprising the carrier wave, a band pass filter 86communicative with the preamplifier 84 configured to filter out unwantednoise in the transmission, and an analog to digital converter (ADC) 88communicative with the band pass filter 86 to convert the analog carrierwave into a digital signal. Such preamplifiers, band pass filters, andND converters are well known in the art and thus are not described indetail here. For example, the preamplifier can be a INA118 model fromTexas Instruments, the ADC can be a ADS1282 model from TexasInstruments, and the band pass filter can be an optical band pass filteror an RLC circuit configured to pass frequencies between 0.1 Hz to 20Hz.

The decoder 82 is, in one embodiment, a general purpose computercomprising a central processing unit (CPU and herein referred to as“surface processor”) and a memory having decoder program code executableby the surface processor to perform various decoding functions,including digital signal filtering and separation, digital signalprocessing, digital signal recombination, and digitalsignal-to-telemetry data decoding. Instead of using the surfaceprocessor to perform all of the decoding functions, separate hardwarecomponents can be used to perform one or more of the decoding functions;for example, an application-specific integrated circuit (ASIC) orfield-programmable gate arrays (FPGA) can be used to perform the digitalsignal processing in a manner as is known in the art.

Referring to FIG. 7, the decoder 82 receives the carrier wave that hasbeen digitized, filtered and amplified by the surface receiver 80, andperforms each of the following decoding functions in the followingsequence (these decoding functions are stored as program code on thememory of the computer and are executable by the surface processor):

Digital Filtering (step 90): When each of the different waveforms isencoded by a modulation technique at a unique frequency, the programcode comprises a series of band pass filters that are used to separatedifferent bands (frequency signals) from the carrier wave. Moreparticularly, each band pass filter is configured to pass one of theunique frequency bands corresponding to one of the separate waveformsand severely attenuate all other frequencies, such that the waveformcorresponding to this unique frequency band can be separated from thecarrier wave.

Digital Signal Processing (Step 92): Each separated waveform is in abitstream form and is then subjected to a series of digital processingtreatments known in the art, such as automatic gain control (AGC) tonormalize the signal amplitude, synchronization to find the phase andtiming differences between incoming signals and local oscillationsignals, and demodulation and decoding to recover binary bits. Suchdigital processing treatments are known in the art of digital signalprocessing and thus are not described in detail here. Then, eachseparate waveform is demodulated back into the corresponding measurementdata bitstream using a demodulation technique that is configured tocorrespond specifically to the modulation technique used to encode theseparate measurement data bitstreams into the separate waveforms. Usingthe example shown in FIGS. 8 to 10 but now referring to FIGS. 12 to 13,the demodulation technique will determine the phase of each separatewaveform, and map the waveform back to the symbol it represents, thusrecovering the original data bitstream (FIG. 12 shows the first waveformand FIG. 13 shows the second waveform). Such demodulation techniques arewell known in the art and thus are not described in detail here.

Digital Signal Recombination (Step 94): After demodulation, the separateraw measurement data streams are recombined back into the single rawmeasurement data stream that existed at step 70. Once the single rawmeasurement data bitstream has been recombined, the data can be decodedand viewed on the computer display 20 or manipulated by the operatorinto a useful form for display (step 96).

Instead of recombining all of the separate measurement data streams intothe single measurement data stream, a subset of separate data stream canbe combined for display to the operator, or each individual data streamcan be processed and displayed for the operator.

According to an alternative embodiment and referring to FIG. 6, thesurface receiver 80 and decoder 82 have been modified such that allsignal filtering is performed by the surface receiver 80 on the analogwaveforms, prior to digitization. More particularly, the surfacereceiver 80 employs narrow band hardware filters 98 of differentfrequency ranges to separate out each separate analog waveform from theanalog carrier wave. Multiple ADCs are then used to convert the multipleanalog waveforms into digital signals. In contrast, the surface receiver80 and decoder 82 according to FIG. 5 performs analog signal filteringof the analog carrier wave, as well as digital signal filtering of thedigitized carrier wave. Here, only one ADC is used to convert thecarrier wave into digital form, and digital bandpass filtering isperformed by software rather than by hardware.

Optionally, the decoder 82 can also execute an algorithm whichcompensates for attenuation caused by the low pass filtercharacteristics of the Earth and other conditions of the drill site.This attenuation compensation algorithm can be constructed in accordancewith one of the methods illustrated in FIGS. 14 to 19. In a first methodshown in FIGS. 14 to 16, a frequency sweep waveform 99 having aconsistent reference amplitude and a continuously increasing frequencyover a determined time period can be transmitted by the EM telemetrytool 32 during an idle time in the drilling so that the drilling processis not interrupted. As can be seen in FIG. 14, the reference amplitudeis set to match the amplitude of each separate waveform used by the EMtelemetry tool 32 to produce the combined waveform, and the frequencycan be set to increase at a selected rate. As can be seen in FIG. 15,the frequency sweep waveform received at surface 100 will attenuateafter having travelled through the Earth to the surface receiver 80. Theattenuation rate will tend to increase with increasing frequency,wherein the value of the attenuation rate will vary with the physicalproperties and operating conditions of the drill site. As can be seen inFIG. 16, the amplitude of the received frequency sweep waveform can beplotted over the frequency range to produce an attenuation curve 101.This attenuation curve 101 is stored on the DSP 92 and is applied to thereceived waveforms to compensate for the attenuation caused by theEarth.

According to another embodiment, another frequency sweep waveform 102and an attenuation curve 104 produced from this frequency sweep waveformis shown in FIGS. 17 to 19. In this frequency sweep waveform 102, thefrequency is held constant for a short period of time, then changed indiscrete steps. FIG. 17 shows this frequency sweep waveform astransmitted by the EM telemetry tool 32, and FIG. 18 shows an attenuatedfrequency sweep waveform 103 as received by the surface receiver whichhas been attenuated travelling through the Earth. FIG. 19 shows astepped attenuation curve over frequency. For the frequencies of thefirst and second downhole waveforms shown in FIGS. 8 and 9 (6 and 12Hz), the expected surface amplitude levels would approximately 0.1 and0.03 based on downhole amplitudes of 1.0.

While the present invention is illustrated by description of severalembodiments and while the illustrative embodiments are described indetail, it is not the intention of the applicants to restrict or in anyway limit the scope of the appended claims to such detail.

Additional advantages and modifications within the scope of the appendedclaims will readily appear to those sufficed in the art. The inventionin its broader aspects is therefore not limited to the specific details,representative apparatus and methods, and illustrative examples shownand described. Accordingly, departures may be made from such detailswithout departing from the spirit or scope of the general concept.

What is claimed is:
 1. A electromagnetic (EM) telemetry methodcomprising (a) encoding downhole data into a single data stream; (b)separating the single data stream into a plurality of separate datastreams; (c) converting each separate data stream into a correspondingseparate waveform using a selected digital modulation technique whereinat least one of the frequency and phase of each waveform is assigned aunique value or unique non-overlapping range of values; (d) combiningeach separate waveform into a combined waveform; and (e) transmittingfrom a downhole location, an electromagnetic (EM) telemetry carrier wavecomprising the combined waveform.
 2. A method as claimed in claim 1wherein the selected digital modulation technique is selected from thegroup consisting of: amplitude shift keying (ASK), phase shift keying(PSK), and frequency shift keying (FSK).
 3. A method as claimed in claim2 wherein only the frequency of each waveform is assigned a uniquevalue.
 4. A method as claimed in claim 1 further comprising after (e):(f) receiving at a surface location, the carrier wave, and filteringeach separate waveform from the carrier wave using the assigned uniquevalue of that separate waveform; (g) demodulating each separate waveforminto the corresponding separate data stream; and (h) combining theseparate data streams into the single data stream.
 5. A method asclaimed in claim 4 further comprising decoding the single data streaminto the downhole data and displaying the downhole data.
 6. A method asclaimed in claim 4 further comprising amplifying the carrier wave.
 7. Amethod as claimed in claim 6 further comprising applying a band-passfilter to the carrier wave to remove unwanted frequencies.
 8. A methodas claimed in claim 7 wherein the carrier wave is analog and the step ofdemodulating is applied to the analog carrier wave and the separate datastreams are analog and are converted into digital data streams.
 9. Amethod as claimed in claim 7 wherein the carrier wave is analog and themethod further comprises converting the analog carrier wave into adigital signal.
 10. A method as claimed in claim 4 further comprisingapplying an attenuation compensation curve to the carrier wave, theattenuation compensation curve constructed by transmitting a selectedfrequency sweep waveform from the downhole location, receiving thefrequency sweep waveform at the surface, and plotting the attenuation ofthe frequency sweep waveform over a frequency range.
 11. A method asclaimed in claim 10 wherein the frequency sweep waveform comprises aconsistent reference amplitude corresponding with an amplitude of theseparate waveforms.
 12. A method as claimed in claim 11 wherein thefrequency sweep waveform is transmitted during an idle time during adrilling operation.
 13. A method as claimed in claim 12 wherein thefrequency sweep waveform comprises a continuously increasing frequencyover a selected time period.
 14. A method as claimed in claim 12 whereinthe frequency sweep waveform comprises discrete steps of increasingfrequency over a selected time period.
 15. An electromagnetic (EM)telemetry system comprising: a downhole telemetry tool comprising: (a) agap sub assembly; (b) an EM carrier frequency signal generator forgenerating an EM carrier wave across an electrically isolated gap of thegap sub assembly; (c) an electronics subassembly communicative with thesignal generator and comprising a downhole processor and a memorycontaining an encoder program code executable by the downhole processorto perform a method comprising: (i) encoding downhole data into a singledata stream; (ii) separating the single data stream into a plurality ofseparate data streams; (iii) converting each separate data stream into acorresponding separate waveform using a selected digital modulationtechnique wherein at least one of the frequency and phase of eachwaveform is assigned a unique value or unique non-overlapping range ofvalues; (iv) combining each separate waveform into a combined waveform;and (v) sending control signal to the signal generator to transmit an EMtelemetry carrier wave comprising the combined waveform.
 16. A system asclaimed in claim 15 wherein the downhole telemetry tool furthercomprises a directional and inclination sensor module and a drillingconditions sensor module both communicative with the processor.
 17. Asystem as claimed in claim 16 further comprising: (a) a surface receiverconfigured to receive the carrier wave; (b) a decoder communicative withthe surface receiver and comprising a surface processor and a memorycontaining a decoder program code executable by the surface processor toperform a method comprising: (i) filtering each separate waveform fromthe carrier wave using the assigned unique value of that separatewaveform; (ii) demodulating each separate waveform into thecorresponding separate data stream; and (iii) combining the separatedata streams into the single data stream.
 18. A system as claimed inclaim 17 wherein the decoder program is further executable by thesurface processor to decode the single data stream into the downholedata and transmit the downhole data to a display.
 19. A system asclaimed in claim 17 wherein the surface receiver further comprises anamplifier configured to amplify the received carrier wave.
 20. A systemas claimed in claim 19 wherein the surface receiver further comprises aband pass filter configured to filter out unwanted noise in the receivedcarrier wave.
 21. A system as claimed in claim 19 further comprising ananalog-to-digital converter to convert the received carrier wave into adigital signal.
 22. A system as claimed in claim 20 wherein the surfacereceiver further comprises a narrow band filter corresponding to eachseparate waveform and configured to separate the corresponding separatewaveform from the carrier wave.